I have entries in /sys/class/backlight (right now it's "thinkpad_screen") with a brightness setting. The reported range is 0-7. Setting values to it changes the value, but not the brightness of the screen.

I tried adding "acpi_backlight=vendor" to my kernel line, which just changed the module from acpi_video0 to thinkpad_screen. I still couldn't change the backlight. I changed acpi_osi to Linux, but that didn't change anything either.